1. Field of the Invention
The present invention relates to a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen in a reversible manner, and it also relates to an electrode using said alloy.
2. Description of the Related Art
Storage batteries, which are widely used as power sources in a variety of applications, are typically classified into two general groups of lead-acid storage batteries and alkaline storage batteries. Between the two groups, alkaline storage batteries tend to be more reliable, and can be made smaller and lighter. Small alkaline storage batteries are generally favored for portable electric appliances, while large alkaline storage batteries have been used mainly in conjunction with industrial equipments.
While some alkaline storage batteries use, for example, air electrode or silver oxide electrode for their positive electrode, majority of the positive electrodes are nickel electrodes. Nickel electrodes have been particularly popular since they were reconfigured from a pocket type to a sintered type and became even more popular with the development of hermetic-sealing.
At present, cadmium is most commonly used to form the negative electrode of alkaline storage batteries, however other materials, including zinc, iron, hydrogen, and the like have also been employed.
There is considerable commercial interest in storage batteries that have a higher energy density than batteries currently available. Toward achieving this goal, researchers have investigated nickel-hydrogen storage batteries which incorporate metal hydride, i.e., hydrogen storage alloy electrodes. A number of proposals have been made on the production method of the hydrogen storage electrodes using metal hydrides.
The alloys in these electrodes, or the hydrides form of such alloys, can absorb and desorb hydrogen in a reversible manner, and thus the alloys and the electrodes made from these alloys have come to be known as hydrogen storage alloys and hydrogen storage electrodes (or hydrogen storage alloy electrodes), respectively.
Batteries made with hydrogen storage electrodes have a larger theoretical energy density compared to batteries formed with cadmium electrodes. Also, batteries that employ hydrogen storage electrodes are not susceptible to the formation and subsequent deformation of dendrites, which is a problem with zinc electrodes. These advantageous properties, as well as the promise for a longer cycle life and a reduction in the environmental concerns inherent in zinc- or cadmium- containing electrodes/batteries, have encouraged research into developing alloys suited for hydrogen storage electrodes, particularly negative electrodes for alkaline storage batteries.
Prior art alloys for hydrogen storage electrodes include multi-component alloys such as those of either the Ti-Ni system, or the La- (or Mm- )Ni system (where Mm is a misch metal). The multi-component alloy of the Ti-Ni system is classified as an AB type (where A is La, Zr, Ti or an element with a similar affinity for hydrogen, and B is Ni, Mn, Cr or any other transition metal). When this type of alloy is used as the negative electrode in an alkaline storage battery, the electrode exhibits a relatively large discharge capacity during the initial charging and discharging cycles. However, electrodes comprising these alloys have a disadvantage that they can not maintain their large discharge capacity after repeated charging and discharging cycles, i.e., do not have large saturation discharge capacities.
Another multi-component alloy is of the La- (or Mm- )Ni system, which is classified as an AB.sub.5 type, where A and B are defined as the afore-mentioned in relation to the AB type of alloy. A number of research project have recently been developed on alloys of this system and thus the alloy have been regarded as a relatively promising alloy material for the electrodes, so far. However, the alloys of this system have several disadvantages such that they have a relatively small discharge capacity, that they have insufficient cycle life performances as the electrodes of the batteries, and that their material cost is expensive. Therefore, there has been a demand for novel alloys usable for making hydrogen storage electrodes having a large discharge capacity and a long cycle life.
A Laves phase alloy of an AB.sub.2 -type (where A is an element with a large affinity for hydrogen such as Zr or Ti, and B is a transition metal such as Ni, Mn or Cr) has the potential to overcome many of the shortcomings of the hydrogen storage alloys described above. Electrodes for a storage battery formed from a Laves phase alloy of the AB.sub.2 -type have relatively high hydrogen storing capability and they are expected to have a large discharge capacity and a long cycle life. The alloys of this system have already been proposed in, for instance, those of the Zr.alpha.V.beta.Ni.sub.7 M.delta. system (Japanese Laid-Open Patent No. Sho 64-60,961) and of the A.sub.x B.sub.y Ni.sub.z system (Japanese Laid-Open Patent No. Hei 1-102,855).
Many of the prior art hydrogen storage alloys of the Zr system and the Zr-Ti system contain vanadium (V). The alloys of those systems however suffer a disadvantage that it is difficult to obtain an electrode excellent in high-rate discharging characteristics and that their material cost is relatively expensive.